As is known in the art, in a conventional internal combustion engine for automotive vehicles, a fuel and air mixture is provided in correct proportions, usually stoichiometric proportion, and a spark is used for igniting the air/fuel mixture. The spark is timed in relation to the position of the pistons in the engine cylinders to generate maximum torque while avoiding engine knock, which is auto-ignition of the air/fuel mixture occurring ahead of the progressing flame front. The variables that influence the propensity for engine knock include engine speed, manifold pressure, coolant temperature, intake air temperature, ambient pressure, EGR, humidity, dilution, enthalpy of vaporization of the fuel, and fuel octane. The spark timing which delivers maximum torque while avoiding engine knock is based upon the instantaneous values for these variables stored in a look-up table in the memory of a microprocessor, which forms a part of the electronic engine control system.
The engine control system obtains readings from various sensors whose signals are a measure of the engine operating conditions and generates an appropriate address to the look-up table in ROM. The control system then computes the correct spark advance for each cylinder.
Generally, advancing the spark for each cylinder increases the torque until a point at which maximum torque is achieved, termed MBT, minimum spark advance for best torque. At certain operating conditions, the spark cannot be advanced to MBT without encountering engine knock. This is characterized by an abnormally rapid rise in cylinder pressure during combustion. That rapid rise in pressure is followed by pressure oscillations, the frequency of which is specific to a given engine configuration and cylinder dimension.
A relatively low energy level of knock arguably is beneficial to engine performance, but audible knock may result in vehicle operator dissatisfaction, and excessive knock can damage the engine. A typical control strategy will distinguish between acceptable and unacceptable levels of knock. The engine control will advance the spark until the knock level becomes unacceptable. This is determined empirically. At that point, the control system will reduce the spark advance until an acceptable level of knock is achieved.
A control system of this type requires a knock sensor that responds to engine vibration energy and functions in the spectrum of rapid cylinder pressure oscillations. Accurate control of knock permits the engine to be calibrated closer to the optimum ignition timing.
The degree of knock depends upon the amount of energy available and the rate of combustion of the end gas. Factors that have an effect on the degree of knock include cylinder temperature, manifold pressure, residual burned fraction, air/fuel ratio, spark timing, octane, homogeneity of the air/fuel mixture, cylinder geometry, compression ratio, and the amount of end gas, i.e., unburned mixture when auto ignition occurs.
Since many of these variables change from cycle to cycle and from cylinder to cylinder, the level of knock also changes from cycle to cycle and from cylinder to cylinder. Therefore, knock intensity is a random phenomenon. At light knock, the occurrence of knock is random from cycle to cycle and cylinder to cylinder. Any variable that affects the combustion process or changes the mass, pressure, temperature, or composition of the end gas contributes to knock intensity and rate of occurrence. For example, in any given engine, some cylinders run hotter than other cylinders or due to manufacturing tolerances have a higher compression ratio than the average of the cylinders. These factors can cause one cylinder to knock more readily than other engine cylinders.
As is also known in the art, knock detection systems that include audio transducers, such as an accelerometer for converting audio signals indicative of abnormal engine combustion into an output voltage that can be used by a microprocessor in controlling engine timing to eliminate knock. Examples of these prior art devices are described in U.S. Pat. Nos. 5,347,846 and/or 6,529,817. Typically, an accelerometer is coupled to the engine, such acceleration producing a charge proportional to the level of vibration. This signal is analyzed for occurrence of frequency components that indicate that one or more cylinders are knocking. The vibration sensor (e.g., accelerometer) is mounted on the engine in a multi-cylinder internal combustion engine. The signal that is obtained from the transducer is filtered and sampled. The voltage amplitudes of several samples are compared by a comparator circuit. If a sample that is measured at an instant later than a sample measured earlier in the combustion cycle is greater in magnitude by a predetermined amount, it is assumed that auto ignition or knock is occurring and an appropriate signal is distributed to a fuel enrichment control or to a spark retard control, or to both, until the auto ignition is eliminated. In another system, an accelerometer is used to sample a signal that includes a background noise portion and a portion that represents knock. The portion of the signal that represents background noise is used to develop a bias for the gain of a control transistor. A knock threshold detector responds to a predetermined increase in the amplitude of the portion of the signal voltage that represents knock above the value that represents background noise and then develops an output signal that is used by the microprocessor to adjust spark timing or fuel supply.
At high engine speeds, the signal difference between background noise and knock is too small to reliably detect knock. In the past knock sensors have been disabled at these high engine speeds.